That's complete claptrap. Yes, very recent (last 5-10 years) widget toolkits have started to force use of features that result in bitmaps being sent across the network, but that's hardly a reason to throw out X. And it's essentially a lie to pretend it means, somehow, that X11 doesn't have network transparency.
I find it pretty bad, to be honest, that the same devs who are protesting that X11's network transparency isn't what it could be are:
1. The devs that did this in the first place, refusing to advance the protocol to include the features that GTK3 et al required.
2. Apparently think the solution is getting rid of network transparency altogether.
I'm staggered, to be honest, by the whole thing. I appreciate old code is sometimes awkward to support, but the solution isn't a wholesale replacement of the project. Mozilla's developers may have made many mistakes in their decision to throw out the Netscape code that delayed the release of a great browser for many years, but they were NEVER, NEVER so stupid as to say "Well, we looked at the Netscape source code, and we think the solution is to get rid of HTML. It's too kludgy! I mean, just look at it, it's impossible to add features to it cleanly!"
If we were talking about a rewrite of X.org, nee XFree86.org, nee X86, that'd be one thing. It's probably necessary by now although I'd still say they need to seriously think in terms of refactoring the project. But throwing out the entire protocol because they refused to add the features necessary to make the protocol efficient? Fuck that.